As they are well known, above all and not only, in the industrial vehicles field, sheets or tarpaulins are used to cover the so-called open top boxes or bodies of the vehicles in order to cover, protect, hold in place and therefore to prevent the spilling of the transported materials.
In case of the transport of unstable and loose materials, as for example sand, gravel and such construction materials, the aerodynamic turbulence can take them away from their loading place in the body of the vehicle and release them on the following vehicles.
This causes driving troubles and accident risks due to poor visibility, windshield cracking or breaking in the following vehicles as well as to other drawbacks as the defilement or the eventual pollution of the road surface. Different solutions are already known in the field of the sheets used to cover the vehicles bodies.
Such solutions are complicated, expensive and must be each time realized according to the real dimensions of the vehicle body to be covered, thereby involving long working times and difficult operations for their application.
Moreover, such solutions imply complicated systems which are for example composed by drive shafting to transmit rotary motion and power to the centerings supporting the tarpaulin.
Such shafting are equipped with conical pairs, to transmit their rotary motion to other shafts which are placed in the corners of said open top bodies, and they require reduction gears in order to lower the stress to be applied for moving the whole system.
The above described systems are not easy to operate by manual operations because they are very heavy and produce remarkable frictional forces even though they are equipped with reduction gear. Therefore, such systems require auxiliary actuating means as electric motors or other similar means.
The EP 1 228 912 discloses a device for opening and closing the cover sheets of vehicles, constituted by driving pulleys set in motions by means of a pair of bevel gears. The driving pulleys are mounted both on the same gear and therefore they rotate in the same direction, leading to the fact that the cable ring extending from one of the two pulleys must be crossed in order to make the upper lateral lengths move in the same direction. This has the disadvantage that the tension of the crossed and non crossed cables are not the same with consequent non-uniform distribution of the strengths; furthermore, as a consequence of the crossing of one cable, its direction of entrance into the couple of lateral pulleys (5s and 5′s in FIG. 1) is slanted with respect to the grooving of said pulleys, what could lead to possible slipping outs of the cable.